Various types of electrocautery devices are used for ablating tissue. Typically, such devices include a conductive tip or blade, which serves as one electrode in an electrical circuit that is completed via a grounding electrode coupled to the patient. With sufficiently high levels of electrical energy between the two electrodes, heat is generated which is sufficient to denature proteins within the tissue and cause cell death.
By controlling the energy level, the amount of heat generated and the degree of tissue damage can also be controlled. High levels of voltage can actually cut and remove tissue (i.e., electrosurgery), while lower levels will simply create sufficient heat to cause cell damage, but leave the structure intact (i.e., ablation) and block electrical pathways within the tissue. Irrigation of the electrode(s) with saline or other conductive fluid can decrease the interface impedance, cool the tissue, and allow for a greater lesion depth.
The treatment of chronic atrial fibrillation (AF) requires the creation of numerous linear lesions that pass completely through the thickness of the tissue. Some electrophysiologists have created these lesions using the tip electrode of standard ablation catheters. These catheters were designed to create spot lesions, typically for ablation of specific structures or focal abnormalities. In order to make the linear lesions required to replicate the maze procedure, an electrophysiologist makes a series of focal lesions, and “connects the dots.”
Manufacturers have therefore developed catheters that have a linear array of electrodes along a long axis (i.e., the Amazr, MECCA, and Revcelation catheters). The catheter and the electrodes are positioned in contact with the tissue. Energy is provided to the electrodes either individually or sequentially. Additionally, catheters which incorporate an electrode that can be energized and moved along a length of the catheter have been proposed.
Surgeons have also been able to create linear lesions on the heart using applications of the techniques discussed above. For example, Kottkamp et al. have used a handheld device that creates a series of spot or short (<1 cm) linear lesions. Other investigators have used long, linear unipolar probes to create somewhat longer lesions. Still others have used multi-electrode linear catheters, similar to those described above, to create a series of ablations that together form a linear lesion.
A bipolar system (in which the grounding electrode is in close proximity to the conductive tip) can create narrower and deeper lesions. The grounding electrode can be approximately the same dimension as the conductive tip, and both electrodes can create the lesion.
One bipolar ablation device has integrated the electrode into the jaws of a hemostat-like or forceps-like device. The device can clamp and ablate the tissue inbetween the jaws. In conjunction with a transmurality algorithm, the device creates transmural lesions. However, the device was designed to access the heart via a mid-line sternotomy. In order for the therapy to be considered as a stand-alone, access must be made less invasively. Simply placing the jaws onto an endoscopic handle has certain advantages, but there are significant limitations when trying to manipulate both jaws simultaneously through separate tissue spaces.
A microwave device has been developed that can loop around the posterior of the heart to encircle the pulmonary veins. A right thorocotomy is created at about the fourth intercostal space, and the pericardium is freed behind the superior vena cava and the inferior vena cava. A movable antenna slides within an integral sheath and discrete sections are ablated in series.
Loop devices do not all use microwave energy. Some investigators have used the radio frequency Cobra device, from Boston Scientific, to encircle the veins. Epicor has initiated clinical studies of a similar device that uses high frequency ultrasonic (HIFU) energy to generate heat within the tissue. Although easy to position within the patient, a significant limitation of these types of loop devices is the ability to reliably create a transmural lesion. Blood is constantly flowing within the heart chamber, and the blood acts as a heat sink to the energy being deposited within the tissue. Blood flow has the largest cooling effect on the tissue at an area of lowest heating. With epicardial systems, it is almost impossible to heat the endocardial layer sufficiently to create a permanent conduction block.
Other investigators have used a clamping device to compress the tissue between two ablative elements. The ablative elements are connected to rigid members that facilitate this compression. Although this bipolar arrangement is very effective at creating transmural lesions, it tends to be difficult to position within the patient. Manipulation of the rigid elements is problematic and can lead to tissue trauma.